This invention relates to a fuel supply control method for internal combustion engines after starting, and more particularly to a method of this kind which is intended to control a fuel quantity supplied to an internal combustion engine immediately after being started in a hot state.
Conventionally, fuel supply control methods for internal combustion engines after starting have been proposed by the assignee of the present application, wherein air-fuel ratio feedback control is effected by supplying a fuel quantity corrected by the use of an O.sub.2 feedback correction coefficient having its value determined in response to the concentration of an ingredient in exhaust gases sensed by an exhaust gas ingredient concentration sensor arranged in an exhaust system of the engine when the sensor is in an activated state, and an initial value of a fuel increment is set as a function of the engine temperature upon generation of a predetermined control signal immediately after the start of the engine, subsequently the set initial value of the fuel increment is progressively decreased each time the predetermined control signal is generated, and a fuel quantity is calculated by the use of the thus decreased fuel increment and supplied to the engine in synchronism with generation of the predetermined control signal.
One of the above fuel supply control methods is such that the feedback control based upon the O.sub.2 feedback correction coefficient is started when the correction of the fuel quantity by the above fuel increment is terminated, that is, when the fuel increment is decreased to a value of 1.0 (Japanese Provisional Patent Publication (Kokai) No. 58-217747, hereinafter called "Prior Art 1"). Another method is such that when the condition for starting the air-fuel ratio feedback control is fulfilled, the fuel increment is set to 1.0 and the feedback control is started by the use of the O.sub.2 feedback correction coefficient (Japanese Provisional Publication (Kokai) No. 61-234237, hereinafter called "Prior Art 2").
However, these prior art methods have the disadvantage that the fuel supply control for an engine after starting cannot be effected in a proper manner if the temperature of the engine immediately after starting is so high that gas bubbles are contained in the fuel within fuel injection valves to be supplied to the engine. More specifically, in the case where the engine is once stopped and restarted again soon, the temperature within the fuel injection valves is often higher than a value at which gas bubbles can be formed in the fuel. Accordingly, the gas bubbles contained in the fuel are also injected into the intake pipe together with the fuel, resulting in a lean mixture being supplied to the engine. If Prior Art 1 is applied to the engine after starting in such hot state, the air-fuel ratio feedback control is not started until the correction of the fuel quantity by the fuel increment is completed, even after the feedback control starting condition is fulfilled, thereby being unable to prevent the mixture from being leaned due to the gas bubbles contained in the fuel. On the other hand, if Prior Art 2 is applied to the hot restarted engine, the air-fuel feedback control is started immediately upon fulfilment of the feedback control starting condition after the start of the engine, but during the feedback control the correction of the fuel quantity by the fuel increment is not effected. As a result, the value of the O.sub.2 feedback correction coefficient is automatically controlled to an extreme value enriching the mixture so as to make up for an amount of fuel to be increased by the fuel increment as well as for the leaning of the mixture due to the presence of gas bubbles in the fuel. This causes the phenomenon that the mixture supplied to the engine becomes overrich immediately after all the gas bubbles injected together with the fuel and burnt have been emitted, resulting in a delay in bringing the correction coefficient to a proper value, and hence preventing stable driveability of the engine from being attained after the start of the engine.